The present invention relates to a static magnetic field correction method and an MRI (Magnetic Resonance Imaging) system. More specifically, the present invention relates to a static magnetic field correction method for correcting the static magnetic field intensity of the MRI system, a static magnetic field correction method for correcting the static magnetic field homogeneity of the MRI system, and an MRI system capable of suitably performing the static magnetic field correction methods.
In a permanent magnet type MRI system, variations in the magnetic characteristic of a permanent magnet are large. A plurality of small magnets are added so as to correct the variations to be a target static magnetic field intensity.
On the other hand, the static magnetic field homogeneity of the MRI system is very important to ensure sufficient image quality. In particular, an EPI (Echo Plannar Imaging) method which has been developing in recent years is very sensitive to the static magnetic field homogeneity. The static magnetic field homogeneity must be sufficiently high.
For this reason, a shim material or magnetic adjustment plate is used to correct the static magnetic field homogeneity.
A related art static magnetic field intensity correction method which adds a plurality of small magnets has the problem that rapid or fine correction is difficult.
Further, since fluctuations of the magnetic characteristic of an NdFeB magnet due to temperature are large, the static magnetic field intensity is fluctuated by the influence of heat generation of a gradient coil. When a metal mass is moved near the MRI system (for example, a vehicle is passed), the static magnetic field intensity is fluctuated. It is impossible to handle such fluctuations of the static magnetic field intensity.
On the other hand, a related art static magnetic field homogeneity correction method which uses a shim material or magnetic adjustment plate has the problem that rapid or fine correction is difficult.
Therefore, a first object of the present invention is to provide a static magnetic field correction method capable of rapidly and finely correcting the static magnetic field intensity of an MRI system and an MRI system capable of suitably performing the static magnetic field correction method.
A second object of the present invention is to provide a static magnetic field correction method capable of rapidly and finely correcting the static magnetic field homogeneity of an MRI system and an MRI system capable of suitably performing the static magnetic field correction method.
In a first aspect, the present invention provides a static magnetic field correction method including the steps of: providing a magnetic field correction coil in a yoke constructing a magnetic circuit of an MRI system, producing a correction magnetic field by flowing a correction electric current to the magnetic field correction coil, and adding a correction magnetic field to the static magnetic field of an imaging region so as to correct the static magnetic field of the imaging region.
In the magnetic field fluctuation measuring method of the first aspect, a magnetic field correction coil is provided in a yoke, to which a correction magnetic field is added. A correction electric current is adjusted to rapidly and finely correct the static magnetic field intensity and homogeneity.
In a second aspect, the present invention provides the static magnetic field correction method thus constructed, further including the steps of: providing a first magnetic field correction coil in a first yoke, providing a second magnetic field correction coil in a second yoke across an imaging region from the first yoke, producing a first correction magnetic field by the first magnetic field correction coil, and producing a second correction magnetic field having its direction and intensity equal to those of the first correction magnetic field by the second magnetic field correction coil.
In the magnetic field fluctuation measuring method of the second aspect, a pair of yokes at the position where an imaging region is interposed therebetween produce correction magnetic fields in which their direction and intensity are equal to each other. The static magnetic field intensity can be corrected.
In a third aspect, the present invention provides the static magnetic field correction method thus constructed, further including the steps of: providing a first magnetic field correction coil in a first yoke, providing a second magnetic field correction coil in a second yoke across an imaging region from the first yoke, producing a first correction magnetic field by the first magnetic field correction coil, and producing a second correction magnetic field having at least one of its direction and intensity different from those of the first correction magnetic field by the second magnetic field correction coil.
In the magnetic field fluctuation measuring method of the third aspect, a pair of yokes at the position where an imaging region is interposed therebetween produce correction magnetic fields in which at least one of the direction and intensity is different. The static magnetic field intensity and homogeneity can be corrected.
In a fourth aspect, the present invention provides the static magnetic field correction method thus constructed, further including the steps of: collecting FID signals, determining a resonance frequency from the FID signals, determining a frequency difference between the resonance frequency and an RF send/receive system, and deciding a correction electric current from the frequency difference.
In the magnetic field fluctuation measuring method of the fourth aspect, a resonance frequency is measured to decide a correction electric current from a frequency difference. The static magnetic field intensity and homogeneity can be corrected accurately.
In a fifth aspect, the present invention provides the static magnetic field correction method thus constructed, further including the steps of: disposing an NMR probe combining a small phantom with a small coil in the vicinity of an imaging region, sending RF pulses from the small coil to receive FID signals from the small phantom by the small coil, and determining a resonance frequency from the FID signals.
In the magnetic field fluctuation measuring method of the fifth aspect, an NMR probe is used to measure a resonance frequency. During imaging a patient, the static magnetic field intensity and homogeneity can be corrected without affecting the imaging. The magnetic field fluctuation measuring method is particularly suitable for correcting the fluctuations.
In a sixth aspect, the present invention provides the static magnetic field correction method thus constructed, further including the steps of: measuring a temperature of a member constructing the magnetic circuit, determining a resonance frequency from a temperature characteristic, determining a frequency difference between the resonance frequency and a target frequency, and deciding a correction electric current from the frequency difference.
In the magnetic field fluctuation measuring method of the sixth aspect, a temperature is measured to decide a correction electric current from a temperature characteristic. The magnetic field fluctuation measuring method is particularly suitable for correcting the fluctuations due to temperature.
In a seventh aspect, the present invention provides an MRI system comprising: a yoke constructing a magnetic circuit, a magnetic field correction coil provided in the yoke for producing a correction magnetic field, and a power source for the magnetic field correction coil for supplying a correction electric current to the magnetic field correction coil.
The MRI system of the seventh aspect can suitably perform the magnetic field fluctuation measuring method of the first aspect.
In an eighth aspect, the present invention provides the MRI system thus constructed, further including: a first yoke and a second yoke at the position where an imaging region is interposed therebetween, a first magnetic field correction coil provided in the first yoke for producing a first correction magnetic field, a second magnetic field correction coil provided in the second yoke and connected in series with the first magnetic field correction coil so as to produce a second correction magnetic field having its direction and intensity equal to those of the first correction magnetic field, and a power source for the magnetic field correction coil for supplying a correction electric current to a series circuit of the first magnetic field correction coil and the second magnetic field correction coil.
The MRI system of the eighth aspect can suitably perform the magnetic field fluctuation measuring method of the second aspect.
In a ninth aspect, the present invention provides the MRI system thus constructed, further including: a first yoke and a second yoke at the position where an imaging region is interposed therebetween, a first magnetic field correction coil provided in the first yoke for producing a first correction magnetic field, a second magnetic field correction coil provided in the second yoke for producing a second correction magnetic field, a power source for the first magnetic field correction coil for supplying a first correction electric current to the first magnetic field correction coil, and a power source for the second magnetic field correction coil for supplying a second correction electric current to the second magnetic field correction coil.
The MRI system of the ninth aspect can suitably perform the magnetic field fluctuation measuring method of the third aspect.
In a tenth aspect, the present invention provides the MRI system thus constructed, further including correction electric current deciding means which collects FID signals, determines a resonance frequency from the FID signals, determines a frequency difference between the resonance frequency and an RF send/receive system, and decides a correction electric current from the frequency difference.
The MRI system of the tenth aspect can suitably perform the magnetic field fluctuation measuring method of the fourth aspect.
In an eleventh aspect, the present invention provides the MRI system thus constructed, further including an NMR probe combining a small phantom with a small coil disposed in the vicinity of an imaging region, wherein
the correction electric current deciding means sends RF pulses from the small coil to receive FID signals from the small phantom by the small coil, and determines a resonance frequency from the FID signals.
The MRI system of the eleventh aspect can suitably perform the magnetic field fluctuation measuring method of the fifth aspect.
In a twelfth aspect, the present invention provides the MRI system thus constructed, further including:
a temperature sensor for measuring a temperature of a member constructing the magnetic circuit; and
a correction electric current deciding means which determines a resonance frequency from the temperature characteristic, determines a frequency difference between the resonance frequency and a target frequency, and decides a correction electric current from the frequency difference.
The MRI system of the twelfth aspect can suitably perform the magnetic field fluctuation measuring method of the sixth aspect.
According to the static magnetic field correction method and the MRI system of the present invention, the static magnetic field intensity and the static magnetic field homogeneity can be corrected rapidly and finely.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.